In general, wettability of a surface of a material to liquid can be measured by measuring a contact angle of the surface.
The wettability is a mutual interaction (adsorption of the liquid to the surface of a solid) between the surface of the material and liquid molecules, which is a competitive state between an adhesive force between the solid and the liquid and a cohesive force between the liquid molecules.
If the cohesive force between the liquid molecules is stronger than the adhesive force between the solid and the liquid, the wettability is low, and if the cohesive force between the liquid molecules is lower than the adhesive force between the solid and the liquid, the wettability is high.
If the liquid is water, the surface having high wettability is called as hydrophilic surface, and the surface having low wettability is called as hydrophobic surface. The surface having the contact angle higher than 90 degrees is called as a hydrophobic surface, and the surface having the contact angle greater than 150 degrees is called as superhydrophobic surface.
Mostly, the hydrophobicity of the surface is fixed according to chemical properties of the surface itself, and a micro/nano size structure of the surface.
Since the material having the hydrophobic surface is excellent in self-cleaning, anti-fogging, reduction of friction caused by fluid, and so on, the material having the hydrophobic surface is widely used in industrial fields that require such effects.
As W. Barthlott and C. Neinhuis report kinds of plant leaves living in the nature having the superhydrophobicity, various surface shapes of the plant leaves, and various phenomena caused by the various surface shapes of the leaves in 1997, introduced recently, there are many methods for forming a pattern of surface having the superhydrophobicity by changing structural characteristics of the surface, which is called as biomimetics.